Method for producing 1,6-hexanediol and 6-hydroxycaproic acid or their esters

ABSTRACT

A process for the preparation of 1,6-hexanediol and 6-hydroxycaproic acid or esters thereof by catalytic hydrogenation of adipic acid, adipic acid monoesters or adipic acid diesters or streams of starting material which contain adipic acid or esters thereof as essential constituents, in which the bottom product obtained in the distillation of the hydrogenation product, following removal of the hexanediol and hydroxycaproic acid or esters thereof, and essentially comprises oligomeric esters of 6-hydroxycaproic acid, is recycled to the hydrogenation, and the resulting mixture of starting material and recycle stream is reacted at from 100 to 300°C., at from 10 to 300 bar in the liquid phase and at a molar ratio of carboxyl groups to be hydrogenated to hydrogen in the reactor of from 1:5 to 1:100 on hydrogenation catalysts is described.

The invention relates to a process for the improved preparation of1,6-hexanediol and 6-hydroxycaproic acid and esters thereof startingfrom adipic acid or mono- and diesters thereof or hydrocarbon streamswhich comprise adipic acid or mono- and diesters thereof and6-hydroxycaproic acid or esters thereof, by catalytic hydrogenation ofthe acids and/or of the esters and recycling of the dimeric andoligomeric compounds which are formed as the bottom product followingdistillation of the hydrogenation product.

U.S. Pat. No. 2,066,533 discloses the catalytic partial hydrogenation ofdicarboxylic acids and esters thereof to the correspondinghydroxycarboxylic acids or lactones thereof, without producingsignificant amounts of diols.

EP 724 908 A1 discloses the hydrogenation, on (modified) Raney noblemetal catalysts, of adipic acid or esters thereof to 1,6-hexanediol and6-hydroxycaproic acid or esters thereof.

JP 49 132 003 discloses the hydrogenation of adipic acid on Mo/Co/SiO₂catalysts to give 1,6-hexanediol and 6-hydroxycaproic acid.

The abovementioned processes have the disadvantage that the productmixtures which are produced during the hydrogenation, ie. alcohols andcarboxylic acids, contain dimeric and oligomeric esters.

Esters of adipic acid with hexanediol and hydroxycarboxylic acid andesters of hydroxycaproic acid and hexanediol may be mentioned by way ofexample (these esters are referred to below as dimers). These esters areunavailable for further use to produce 6-hydroxycaproic acid(6-hydroxycaproate) and 1,6-hexanediol, and have, following removal ofthe desired products by distillation, to be removed in a further processstep, such as eg. hydrolysis with water, which is an equilibriumreaction and does not produce complete conversion. The abovementionedprocesses are thus only economical to a limited extent.

It is an object of the invention to overcome this prior artdisadvantage.

Surprisingly, we have found that it is possible to significantlyincrease the overall yield of 6-hydroxycaproic acid or esters thereofand 1,6-hexanediol using a process for the preparation of 1,6-hexanedioland 6-hydroxycaproic acid or esters thereof by catalytic hydrogenationof adipic acid, adipic acid monoesters or adipic acid diesters orstreams of starting materials which contain adipic acid or estersthereof as essential constituents, if the bottom product which isobtained in the distillation of the hydrogenation product, followingremoval of the hexanediol and hydroxycaproic acid or esters thereof, andessentially comprises oligomeric esters of 6-hydroxycaproic acid, isrecycled to the hydrogenation and the resulting mixture of startingmaterial and recycle stream is reacted at from 100 to 300° C. and atfrom 10 to 300 bar in the liquid phase and at a molar ratio of carboxylgroups to be hydrogenated to hydrogen in the reactor of from 1:5 to1:100 on hydrogenation catalysts.

It was surprising that the recycled C₆-dimers and C₆-oligomers can bereacted under the reaction conditions of the hydrogenation of themonomeric acids and esters thereof, without a direct increase in thelevel of dimers, oligomers and byproducts, to give 1,6-hexanediol and6-hydroxycaproic acid and esters thereof, and that the selectivity ofthe reaction is not impaired. It was also surprising that the usefullife of the catalyst is not impaired by the recycling, since it wouldhave been assumed that some dimeric and oligomeric compounds deposit onthe catalyst, impairing its activity and selectivity as a result.

The alcohol component of the esters of adipic acid and 6-hydroxycaproicacid is preferably methanol, ethanol, n-propanol, i-propanol, n-butanol,i-butanol and n-pentanol. A particular ester of hydroxycaproic acid isthe internal ester caprolactone. The starting material used for thehydrogenation is adipic acid or mono- and/or diesters thereof. Thisstarting material may also contain further C₆ compounds, eg.6-hydroxycaproic acid or esters thereof. It is also possible for furthernon-C₆ compounds which do not impair the novel process, eg. acids suchas glutaric acid or succinic acid or esters thereof, to be present. Suchmixtures are described, for example, in DE-A 19 607 953. The novelprocess may use, for example, the stream from stage 4 or the top productstream from stage 12 from Example 1.c. of the above patent.

The hydrogenation is preferably carried out in the liquid phase. Thehydrogenation catalysts generally used in the novel process areheterogeneous catalysts, but it is also possible to use homogeneouscatalysts which are suitable for hydrogenating carbonyl groups. They caneither be arranged as fixed bed catalysts or be employed in mobile form,for example in a fluidized bed reactor. Examples of hydrogenationcatalysts for this purpose are described, for example, in Houben-Weyl,Methoden der Organischen Chemie, Volume IV/1c, pages 16 to 26.

Of the hydrogenation catalysts to be used according to the invention,preference is given to those containing one or more elements from groupsIb, VIb, VIIb and VIIIb, and IIIa, IVa and Va, of the Periodic Table ofthe Elements, in particular copper, chromium, rhenium, cobalt, rhodium,nickel, palladium, iron, platinum, indium, tin and/or antimony.Particular preference is given to catalysts containing copper, cobaltand/or rhenium.

The catalysts employed in the novel process may be, for example,precipitated catalysts. Catalysts of this type can be prepared byprecipitating their catalytically active components from solutions ofsalts thereof, in particular from solutions of their nitrates and/oracetates, for example by adding solutions of alkali metal hydroxideand/or alkaline earth metal hydroxide and/or alkali metal carbonateand/or alkaline earth metal carbonate, eg. sparingly soluble hydroxides,oxide hydrates, basic salts or carbonates, then drying the resultingprecipitates and subsequently converting them by calcination at, ingeneral, from 300 to 700° C., in particular from 400 to 600° C., intothe corresponding oxides, mixed oxides and/or mixed valency oxides,which are reduced, and converted into the actual catalytically activeform, by treatment with hydrogen or hydrogen-containing gases, generallyat from 50 to 700° C., in particular from 100 to 400° C., to give therelevant metals and/or oxides of lower oxidation state. This reductionis generally continued until water is no longer formed. To prepareprecipitated catalysts containing a carrier material, the catalyticallyactive components can be precipitated in the presence of the relevantcarrier material. However, it is also possible advantageously for thecatalytically active components to be precipitated simultaneously withthe carrier material from the relevant salt solutions. The hydrogenationcatalysts preferably employed in the novel process are those containingthe hydrogenation-catalyzing metals or metal compounds deposited on acarrier material. Apart from the abovementioned precipitated catalysts,which also comprise a carrier material in addition to the catalyticallyactive components, suitable carrier materials for the novel process are,in general, those in which the components catalyzing the hydrogenationhave been applied to a carrier material, for example by impregnation.

The way in which the catalytically active metals are applied to thecarrier is generally not critical and can be brought about in variousways. The catalytically active metals can be applied to these carriermaterials for example by impregnation with solutions or suspensions ofthe salts or oxides of the relevant elements, drying and subsequentreduction of the metal compounds to the corresponding metals orcompounds of a lower oxidation state by means of a reducing agent,preferably using hydrogen or complex hydrides. Another potential way ofapplying the catalytically active metals to these carriers consists inimpregnating the carriers with solutions of salts which readily undergothermal decomposition, eg. with nitrates, or complex compounds whichreadily undergo thermal decomposition, eg. carbonyl or hydrido complexesof the catalytically active metals, and heating the carrier impregnatedin this way to from 300 to 600° C. for thermal decomposition of theadsorbed metal compounds. This thermal decomposition is preferablycarried out under a protective gas atmosphere. Examples of suitableprotective gases are nitrogen, carbon dioxide, hydrogen or the inertgases. The catalytically active metals can furthermore be deposited onthe catalyst carrier by vapor deposition or by flame spraying. Thecontent of catalytically active metals in these supported catalysts isnot in principle critical for success of the novel process. It isself-evident to the person skilled in the art that higher contents ofcatalytically active metals in these supported catalysts may result inhigher space-time conversions than lower contents. The supportedcatalysts generally used comprise from 0.1 to 90% by weight, preferablyfrom 0.5 to 40% by weight, of catalytically active metals, based on theentire catalyst. Since these contents refer to the entire catalystincluding carrier material, but different carrier materials have verydifferent specific gravities and specific surface areas, lower or highercontents than these are also possible without this necessarily having adisadvantageous effect on the result of the novel process. It is, ofcourse, also possible to apply a plurality of catalytically activemetals to the particular carrier material. Furthermore, thecatalytically active metals can be applied to the carrier for example bythe process of DE-A 2 519 817, EP-A 147 219 and EP-A 285 420. Thecatalytically active metals are present in the catalysts disclosed inthe abovementioned publications as alloys which are produced by thermaltreatment and/or reduction after, for example, impregnation with a saltor complex of the abovementioned metals.

Activation both of the precipitated catalysts and of the supportedcatalysts can also take place in situ at the start of the reaction bythe hydrogen which is present, but these catalysts are preferablyactivated separately before being used.

Suitable carrier materials are generally the oxides of aluminum andtitanium, zirconium dioxide, silicon dioxide, clays, such asmontmorillonites, silicates, such as magnesium or aluminum silicates,zeolites, such as ZSM-5 or ZSM-10 zeolites, or activated carbon.Preferred carrier materials are aluminum oxides, titanium dioxides,silicon dioxide, zirconium dioxide and activated carbon. It is, ofcourse, also possible to use mixtures of various carrier materials ascarrier for the catalysts which can be used in the novel process.Examples of heterogeneous catalysts which can be employed in the novelprocess are the following:

cobalt on activated carbon, cobalt on silicon dioxide, cobalt onaluminum oxide, rhenium on activated carbon, rhenium on silicon dioxide,rhenium/tin on activated carbon, rhenium/platinum on activated carbon,copper on activated carbon, copper/silicon dioxide, copper/aluminumoxide, copper chromite, barium copper chromite, copper/aluminumoxide/manganese oxide, copper/aluminum oxide/zinc oxide, and thecatalysts disclosed in DE-A 3 932 332, U.S. Pat. No. 3,449,445, EP-A 44444, EP-A 147 219, DE-A 3 904 083, DE-A 2 321 101, EP-A 415 202, DE-A 2366 264, EP 0 552 463 and EP-A 100 406.

Particularly preferred catalysts contain at least one of the metalscopper, cobalt or rhenium.

The novel process can advantageously be carried out continuously using,for example, tubular reactors in which the catalyst is advantageouslyarranged in the form of a fixed bed.

According to the invention, the molar ratio of groups to behydrogenated, ie. carboxyl group either as acid group or ester group, tohydrogen in the reactor is between 1:5 and 1:100, preferably between 1:7and 1:70. The requisite reaction pressure is above 10 bar, preferably100-300 bar, particularly preferably 150-300 bar. The reactiontemperatures are in the range 100-300° C., preferably 130-270° C.,particularly preferably 160-240° C. Between 0.05 kg and 5 kg, preferably0.1 and 3 kg, particularly preferably 0.2 and 1.5 kg, of startingmaterial per hour are passed over the hydrogenation catalyst per part byvolume of catalyst (=space velocity).

A solvent is not necessary, although one can be used. Suitable solventsinclude water or the alcohol of esters used, such as methanol, ethanol,etc. Water is used preferentially when the free acid is used as solvent.

The ratio of the desired products to one another can vary within wideranges. In the case of relatively low conversions, 6-hydroxycaproic acidor esters thereof dominates. Economical and therefore preferred molarratios of 1,6-hexanediol to hydroxycaproic acid or esters thereof arefrom 1:5 to 100:1, preferably 1:2 to 100:1, particularly preferably 1:1to 100:1.

The ratio can be influenced, for example, by choice of temperature,pressure, space velocity or residence time. The lower the temperature,pressure and residence time and the higher the space velocity, thehigher the proportion of hydroxycarpoic acid or esters thereof.

The desired products 1,6-hexanediol and 6-hydroxycaproic acid or estersthereof are obtained in a manner known per se, eg. by distillation underreduced pressure, for example at 10-500 mbar, preferably 15-100 mbar andstill temperatures of 100-250° C., preferably 120-220° C. This producesthe mixture, which can be recycled to the hydrogenation, whose mainconstituent is, inter alia, more than 50% by weight, preferably morethan 60% by weight and, in particular, more than 70% by weight, ofoligomeric 6-hydroxycaproate, as the bottom product of the distillationcolumn. Particularly when the still temperature is above 150° C. and themolar proportion of hydroxycaproic acid or esters thereof to hexanediolin the hydrogenation mixture is greater than 1:5, it is sensible to keepthe residence time of the mixture in the still of the distillationcolumn, provided the latter is operated continuously, as short aspossible, for example less than 2 hours, preferably less than one hour,particularly preferably less than 0.5 hour, in order to avoid highmolecular weight esters from forming in the bottom product, which hinderrecycling since they have high melting points which can be above 200° C.

The unreacted adipic acid or esters thereof can of course likewise berecycled to the hydrogenation.

The recycled bottom product can be recycled batchwise, but when theprocess is carried out on an industrial scale, is preferably recycledcontinuously. The recycled product stream can be mixed into the freshfeed upstream of the reactor, or fed directly into the reactor as asecond feed. If the hydrogenation is carried out, for example, usingprimary and secondary reactors, the recycled stream can be introducedinto either or both. Although there is usually no increase in the levelof dimers and oligomers and, in some instances, in byproducts, it may benecessary to discharge a small amount of the bottom product from thedistillation. For this purpose, a batchwise procedure generally involvesrecycling all of the distillation bottom product until there is anincrease in the level of undesired products and then, at appropriateintervals, discharging part of or an entire batch of bottom product.Carrying out the process continuously involves recycling at least mostof the distillation bottom product and continuously discharging, asnecessary, a relatively small amount of the bottom product.

1,6-Hexanediol is a desired monomer building block which is mainly usedin the polyester and polyurethane sector. 6-Hydroxycaproic acid andesters thereof are intermediates in the preparation of caprolactone fromwhich polycaprolactones are obtained. The novel process is described inmore detail with reference to the following Examples, but is not limitedthereto. The analytical results given were determined by gaschromatography using an internal standard and are % by weight.

EXAMPLE 1

25 ml of T 4489 Cu catalyst from Sud-Chemie which had been activatedbeforehand in a hydrogen stream were introduced into a 25 ml tubularreactor. The reactor was brought to 220 bar and 175° C. by means ofexternal oil heating. A fresh gas stream of 100 liters (STP)/h ofhydrogen was established. As a result, 22.5 ml/h of dimethyl adipatewere continuously hydrogenated on downward flow through the catalystbed. Under the reaction conditions the molar ratio of ester groups to behydrogenated to hydrogen in the reactor was 1:25. After a running intime of 12 h, the product contained (calculated on a methanol-freebasis) 35% of dimethyl adipate, 10% of methyl 6-hydroxycaproate, 30% of1,6-hexanediol, 10% of esters of 6-hydroxycaproic acid and1,6-hexanediol and 14% of esters of adipic acid, 1,6-hexanediol andmethanol. The remainder was other mixed esters and oligomers. Dimethyladipate, methyl 6-hydroxycaproate and 1,6-hexanediol were distilled offfrom this mixture. The residue which remained was mixed with freshdimethyl adipate in the ratio 1:5 and hydrogenated again over the samecatalyst under the abovementioned conditions. It is found that theresulting hydrogenation product corresponds to the composition which wasobtained without admixture of the distillation bottom product. This factdid not change even after 5 recycles.

EXAMPLE 2

Hydrogenation was carried out as described in Example 1 over a catalystwhich comprised 1% of Re and 1% of Pt on aluminum oxide (prepared byapplying PtO₂ and Re₂O₇ to Al₂O₃ and subsequent reduction in a hydrogenstream). At a hydrogenation temperature of 163° C. the product comprised(calculated on a methanol-free basis) 1% of n-hexanol, 2% of1,6-hexanediol, 10% of methyl 6-hydroxycaproate and 73% of dimethyladipate. The remainder largely comprised dimeric and oligomeric mixedesters, predominantly those of 1,6-hexanediol. After n-hexanol, dimethyladipate, methyl 6-hydroxycaproate and 1,6-hexanediol had been removed bydistillation, the distillation bottom product which remained was mixedwith fresh dimethyl adipate in the ratio 1:6 and rehydrogenated. Thecomposition of the hydrogenation product remained practically unchanged.

We claim:
 1. A process for the preparation of 1,6-hexanediol and6-hydroxycaproic acid or esters thereof with monoalcohols by catalytichydrogenation of adipic acid, adipic acid monoesters or adipic aciddiesters or streams of starting material which contain adipic acid oresters thereof as essential constituents, which comprises recycling thebottom product which is obtained in the distillation of thehydrogenation product, following removal of the hexanediol andhydroxycaproic acid or esters thereof, and essentially comprisesoligomeric esters of 6-hydroxycaporic acid to the hydrogenation, andreacting the resulting mixture of starting material and recycle streamat from 100 to 300° C. and at from 10 to 300 bar in the liquid phase andat a molar ratio of carboxyl groups to be hydrogenated to hydrogen inthe reactor of from 1:5 to 1:100 on hydrogenation catalysts.
 2. Aprocess as claimed in claim 1, which comprises carrying out thehydrogenation on a fixed bed catalyst.
 3. A process as claimed in claim1, which comprises carrying out the hydrogenation on hydrogenationcatalysts which comprise, as hydrogenation-active constituents, one ormore elements of groups Ib, VIb, VIIb and VIIIb, and IlIa, IVa and Va,of the Periodic Table of the Elements.
 4. A process as claimed in claim1, which comprises carrying out the hydrogenation on hydrogenationcatalysts which comprise, as hydrogenation-active constituents, one ormore elements selected from the group consisting of copper, chromium,rhenium, cobalt, rhodium, nickel, palladium, iron, platinum, indium, tinand antimony.
 5. A process as claimed in claim 4, which comprisescarrying out the hydrogenation using supported catalysts.
 6. A processas claimed in claim 1, which comprises using a hydrogenation catalystcontaining at least copper, cobalt or rhenium.
 7. A batch process asclaimed in claim 1, in which all of the distillation bottom product ofthe hydrogenation product is repeatedly recycled, and only when there isan increase in the level of byproducts is some of the bottom productdischarged.
 8. A continuous process as claimed in claim 1, in which atleast most of the distillation bottom product is continuously recycledand, as necessary, a relatively small amount of the bottom product iscontinuously discharged.
 9. A process as claimed in claim 1, wherein, byadjusting the hydrogen excess, the residence time and, if necessary,other reaction parameters, a molar ratio of 1,6-hexanediol to6-hydroxycaproic acid or esters thereof in the hydrogenation productbetween 1:5 to 20:1 is established.
 10. A process as claimed in claim 1,wherein, said monoalcohols are selected from the group consisting ofmenthanol, ethanol, n-propanol, i-propanol, n-butanol, i-butanol andn-pentanol.
 11. A process as claimed in claimed 1, wherein said bottomproduct consists of more than 50% by weight oligomeric6-hydroxycaproate.